The present invention relates to an improved corrosion-resistant, surface-coated steel sheet. More particularly, the invention relates to a corrosion-resistant steel sheet coated with a multilayer organic-inorganic composite coating which has good weldability and formability in addition to good corrosion-preventing properties even if a protecting paint coating is injured and which is especially suitable for use as automobile panels including outer panels.
In recent years, requirements for corrosion resistance of steel sheets for use as automobile panels have become increasingly strict. For example, such steel sheets are required to resist perforative corrosion for 10 years and surface rusting for 5 years in north America and Europe where severe corrosive conditions are created in winter since rock salt is generally spread on roads in order to prevent them from freezing.
Under these circumstances, surface-coated, weldable steel sheets have been substituted for conventional cold-rolled steel sheets to fabricate inner and outer panels of automobiles. For this purpose, steel sheets plated with zinc or a zinc alloy have been frequently used, but they do not have adequate corrosion resistance unless the zinc or zinc alloy plating has an extremely large thickness. However, such thick plating adversely affects the press-formability of the plated steel sheet and powdering and flaking of the plating tend to occur during press-forming of the sheet into the shape of an automobile panel.
Japanese Patent Application Kokai No. 58-6995(1983) describes a Zn-Ni alloy-plated steel sheet having on at least one surface thereof a first (lower) Zn-Ni alloy plating layer of a (.eta.+.gamma.) dual phase containing 2-9 wt % of Ni and having a thickness of 0.05-2 .mu.m and a second (upper) Zn-Ni alloy plating layer of a .gamma. single phase containing 10-20 wt % of Ni and having a thickness of 0.2-10 .mu.m wherein the thickness ratio of the first layer to the second layer is from 1:5 to 1:100. The duplex Ni-Zn plating is effective to prevent cosmetic corrosion and surface rusting after paint coating.
The thickness of the upper plating layer which has a higher Ni content and which is more brittle than the lower plating layer is much greater than that of the lower plating. Therefore, in a low-temperature chipping test which simulates the situation that pebbles hit against a car body in winter, the plating will be peeled away or chipped off over a large area, leading to a decrease in ultimate corrosion resistance. Furthermore, the presence of the thick, high-Ni alloy upper layer which is relatively noble is considered to accelerate corrosion of the relatively thin, low-Ni alloy lower layer and also increases the costs of the plated steel sheet, since Ni is rather expensive.
Another type of corrosion resistant, surface-coated steel sheet which has been developed is based on a zinc or zinc-alloy plated steel sheet and has a chromate film and an organic coating thereon. Thus, this type of coated steel sheet has a multilayer inorganic-organic composite coating on at least one surface.
A typical example of such a surface-coated steel sheet is known as Zincrometal.RTM.. It has an organic coating of a zinc-rich primer. However, it does not have sufficient corrosion resistance and tends to suffer from powdering of the coating during press-forming due to the presence of a large amount of Zn powder in the uppermost organic coating.
Surface-coated steel sheets having a chromate film and an organic composite silicate coating on a zinc or zinc alloy-plated steel sheet have been disclosed in Japanese Patent Application Kokai Nos. 57-108212(1982), 58-224174(1983), and 60-174879(1985). These surface-coated steel sheets have improved resistance to powdering since the organic coating does not contain metallic powder. However, their corrosion resistance still does not reach a satisfactory level.
Many attempts have been made to modify one or more of the plating, chromate, and organic coating layers of the above-described multilayer surface-coated steel sheets.
Japanese Patent Application Kokai No. 58-210192(1983) discloses a surface-coated steel sheet plated with a Ni-Zn alloy of the .gamma. single phase containing 9-20 wt % Ni and having a chromate film and a conductive material-containing organic coating on the plating layer. Japanese Patent Application Kokai No. 58-210190(1983) discloses a similar surface-coated steel sheet in which the plating layer is a duplex plating consisting of a lower .gamma.-phase Ni-Zn alloy layer and an upper Fe-Zn alloy plating containing 10-40 wt % Fe.
Japanese Patent Application Kokai No. 61-84381(1986) describes a surface-coated steel sheet plated with a .eta.-phase Ni-Zn alloy containing 1-3 wt % Ni and having thereon a chromate film and a polymer coating.
Japanese Patent Application Kokai No. 63-203778(1988) describes a surface-coated steel sheet plated with a zinc or zinc alloy in which fine particles of an insoluble metal compound such as an oxide, carbide, nitride, boride, phosphide, or sulfide of Si, Al, Fe, or the like are dispersed in order to modify the properties of the plating layer and which has a chromate film and an organic coating layer on the plating.
Japanese Patent Application Kokai No. 62-268635(1987) describes a surface-coated steel sheet having a zinc-based plating layer, a colloidal silica-containing chromate film, and a thin clear film of a polyhydroxypolyether resin which may contain a chromate pigment. Japanese Patent Application Kokai No. 1-80522(1989) discloses a similar surface-coated steel sheet in which the uppermost clear film is formed from a coating composition based on an epoxy or modified epoxy resin and containing at least one additive selected from inorganic fillers and cross-linking agents.
These various modifications of one or more of the layers proposed in the prior art can improve the corrosion resistance of surface-coated, weldable steel sheets for use as automobile panels. However, the improved corrosion resistance is mainly intended to increase resistance to perforative corrosion which occurs on a bare plated surface having no paint coating. Therefore, the above-mentioned type of surface-coated steel sheets having an inorganic-organic composite coating have been used for inner panels of automobiles which are usually partially covered with a paint coating. The cosmetic corrosion resistance of such surface-coated steel sheets after it has been covered with a paint coating is not satisfactory if the paint coating is injured.
As the requirements for corrosion resistance of automobile panels become stricter, it has been attempted to employ surface-coated steel sheets not only as inner panels but also as outer panels in automobiles. Automobile outer panels which are completely covered with a surface paint coating which is typically performed by electrodeposition coating of a primer followed by intercoating of a surfacer and topcoating are often injured accidentally, for example, by a hit of pebbles or chippings and hence they are required to withstand corrosion even if the surface paint coating is chipped or otherwise injured. Therefore, they must have good resistance to cosmetic corrosion which occurs in chipped areas of outer panels, i.e., those areas in which the surface coating is chipped off.
Recently, cosmetic corrosion resistance in chipped areas has become a requisite property for automobile inner panels as well, since they are usually covered with a paint coating at least partially and the coating may possibly be injured or chipped during conveying, transportation, and press-forming. Therefore, cosmetic corrosion resistance also contributes to improved corrosion resistance in automobile inner panels.
Accordingly, there is an extensive demand for surface-coated steel sheets having improved resistance to corrosion, particularly to cosmetic corrosion in chipped areas.